1. Field of the Invention
The invention relates to an AC/DC converter comprising a resonance converter.
2. Description of the Related Art
Such AC/DC converters are used for converting an AC voltage into a DC voltage in, for example, television sets or discharge lamps, in the form of switched-mode power supplies so as to convert an AC line voltage into a DC supply voltage.
An AC/DC converter, which forms a load on a public AC power grid, is subject to particular requirements as regards the current which may be taken from the AC power grid. For example, the current taken up by the AC/DC converter may customarily have only a limited harmonic part, i.e., the AC/DC converter should, in essence, represent an ohmic resistance. Such requirements are further specified in, for example, IEC 1000-3-2.
German Offenlegungsschrift DE 198 24 409 A1, corresponding to U.S. Pat. No. 6,046,914, discloses an AC/DC converter comprising a resonance converter which directly connects an up-converter comprising purely passive components to the output of a half-bridge. The publication by W. Chen, F. C. Lee and T. Yamauchi xe2x80x9cAn improved xe2x80x98Charge Pumpxe2x80x99 electronic ballast with low THD and low crest factorxe2x80x9d, IEEE APEC ""96 Proceedings, pp. 622-627, contains further possibilities of realizing such an arrangement. On the other hand, J. Wustehube, Schaltnetzteile, second revised edition, p. 139 ff., describes a bridge rectifier circuit with a switch-over device by means of which the bridge rectifier circuit is adapted to the respectively present AC line voltage (110-127 volts, for example, in the USA or 220-240 volts, for example, in Europe), so that the generated DC voltage has substantially the same values irrespective of the applied AC line voltage.
It is an object of the invention to provide an AC/DC converter comprising a resonance converter which is highly cost-effective and suitable for use with different AC line voltages from different AC power grids. The current taken up by the AC/DC converter is then to have only a limited harmonic part and work, in essence, as an ohmic resistance.
This object is achieved by an AC/DC converter comprising a first rectifier arrangement for generating a rectified AC voltage from a first AC voltage present on an input of the AC/DC converter, a first smoothing capacitor arrangement for smoothing the rectified AC voltage thereby forming a smoothed rectified AC voltage, a bridge circuit including a first, second, third and fourth switching element for converting the smoothed rectified AC voltage into a further AC voltage, a resonance converter comprising resonant circuit elements for converting the further AC voltage into a DC output voltage available on an output of the AC/DC converter, a control circuit for controlling the switching elements of the bridge circuit, wherein a first mode is provided in which the bridge circuit is operated as a half-bridge circuit by changing the switching states of the first and second switching elements and the switching states of the third and fourth switching elements are not changed, and wherein a second mode is provided in which the bridge circuit is operated as a full-bridge circuit by changing the switching states of all four switching elements, an arrangement working as an up-converter connected between the first rectifier arrangement and the first smoothing capacitor arrangement, said arrangement comprising at least one diode and a coupling capacitor, said coupling capacitor being coupled on one end to a junction point between the first rectifier arrangement and the at least one diode, and being coupled, on the other end, to a point inside the resonance converter, whereby, during the operation of the AC/DC converter, a potential modulated with the operating frequency of the resonance converter is fed back.
By using the two modes, the ratio of the DC output voltage to the first AC voltage present on the input of the AC/DC converter can be adjusted. This adjusting option reduces the requirements of the control circuit and allows the use of the same components for AC/DC converters that are provided for the operation with different AC line voltages, these voltages being present as a first AC voltage on the AC/DC converter, or for different DC output voltages. This leads to a considerable saving of cost of the AC/DC converter.
The use of the arrangement coupled to the resonance converter and working as an up-converter, leads to a further reduction of the harmonic part which is fed back into the AC power grid. Furthermore, the arrangement working as an up-converter provides a stabilization of the smoothed rectified AC voltage. This again reduces the requirements of the control circuit.
Variants of the invention, in which, for example, the arrangement working as an up-converter comprises a further diode connected in series between the first rectifier arrangement and the junction point to which the coupling capacitor is coupled, and an inductance connected between this further diode and the junction point of the coupling capacitor, have an advantageous effect on the load of the mains caused by the AC/DC converter, on the practical use of the AC/DC converter or on the construction costs of the AC/DC converter.
The invention, however, also relates to an integrated circuit which integrates at least the control circuit or which integrates at least the control circuit plus the four switching elements of the bridge circuit with one component. Such integration achieves a further reduction of the construction costs.
A further aspect of the invention is that an AC/DC converter according to the invention is particularly suitable for monitors and for television sets, for example, with flat screens. These apparatuses require an exactly controlled and smoothed power supply with a substantially sinusoidal pattern of the line current consumption in accordance with the legal directives.